Rename 'silent' to 'important', to avoid confusion

Hello @don-neufeld, 

Please review the following commits I made in branch 'nick-important'.

f3ddda1a717e457e46aa375b23dd0c10968408e0 (2013-07-17 15:29:36 -0400)
Rename 'silent' to 'important', to avoid confusion


R=@don-neufeld